The change of pitch or variable calibration of the pitches of blades of a turbine engine propeller is one avenue for improving the performance and output of turbine engines under different flight conditions.
It is known turbine engine such as turboprops, for example with despun propellers, referred to by the expressions “open rotor” and “unducted fan”, equipped with these pitch change systems. Turboprops differ from turbojet engines by the use of a propeller outside the nacelle (unducted) instead of an internal fan. The pitch change system may also apply to a turboprop with a single propeller or adapt indifferently to several propellers.
In a turboprop of the open rotor type, a gas-generating part and a propulsion part are aligned and arranged in a stationary cylindrical nacelle supported by the structure of the aircraft. The gas-generating part can be arranged in front of or behind the propulsion part. The propulsion part includes a pair of coaxial and often despun propellers, upstream and downstream, respectively, that are rotated often in opposite directions relative to one another by a turbine, in particular a low-pressure turbine, of the gas-generating part via a reduction gear or a epicyclic gearbox. The propellers extend substantially radially across from the transmission shaft outside the nacelle. In general, each propeller comprises a hub with an outer polygonal ring received rotatably in the stationary nacelle and having radial cylindrical housings distributed on its periphery around the longitudinal axis of the turbine engine. The shafts of the root of the blades are received in the housings of the rings.
The system for changing the pitch of each propeller is installed in the core of the rotary parts, for example with a power cylinder driving the root of the blades for their pitch change rotation. The annular power cylinder includes a cylinder mounted on a flange secured to a stationary part of the turbine engine and a piston connected to the root of the blades by a connecting mechanism. A bearing is arranged between the flange secured to the stationary part and the cylinder of the power cylinder. The movement of the piston following the fluid command of the annular power cylinder ensures the desired angular pivoting of the blades by the connecting mechanism by varying their pitch. Such pitch change systems in turbine engines with despun propellers are known from documents US-A1-20120079808, FR-A1-2980770 and FR-A1-3013325.
One of the difficulties of this architecture consists of converting power arriving from the stationary part into movement on the rotating part of the turbine engine. Another difficulty lies in the dimensioning and the integration of the various elements into this rotating part. In particular, the flange secured to the stationary part of the turbine engine and the bearing induce a significant lever arm that involves dimensioning difficulties. Furthermore, the power cylinder is subject to substantial radial forces from the rotary part that involve deformations of the power cylinder as well as sealing problems. These forces are multiplied when the power cylinder is such that it plays a structural role. The dimensioning difficulties are also due to limited space constraints to arrange the pitch change system, the arrangement and balancing of the rolling bearings, and the passages of the compartments necessary for the operation of the power cylinder of each propeller. This results in a complex arrangement and additional equipment that affect the drag.